Synchronization of the gram-positive B. subtilis has been used as a tool for the studies of the chromosome replication pattern during the cell division cycle. SR22 spo MOT minus, a derivative of B. subtilis 168trp is being analyzed at a variety of growth rates by selecting newborn cells from exponential cell population bound onto a Millipore nitrocellulose filter. We have found that the length of the C+D periods is comparable to that of the gram-negative Escherichia coli B/r, but both DNA content and cellular mass of the B. subtilis strain are 2 to 3-fold larger. The possibility that B. subtilis cells are diploid at all growth rates will be explored by determining the DNA content of vegetative cells, of spores and of sporangia of an isogenic spo plus revertant of B. subtilis SR22. The process of sporulation will be studied through the comparison of the extent of cell division and of the residual DNA replication, in thymine-requiring mutants, during transfer and growth in sporulating media of the spo plus and spo cells.